Stabilized lubricants based on polyethers

The present invention relates to lubricants which are based on polyethers and are stabilized with particular diphenylamine derivatives.

Lubricants based on polyethers are known. They are distinguished by an outstanding lubricating capacity, good viscosity properties, a high flash point, low volatility, a low pour point and little action on metals and sealing materials. They can be employed over a wide temperature range, for example from -20° to +220° C. On the basis of these properties, lubricants based on polyethers are employed, in particular, as hydraulic fluids, brake fluids, metalworking fluids and lubricants for compressors and refrigerating machines and as bearing and gear lubricant oils for units exposed to high thermal and mechanical stresses in the paper, textile and plastics industry.

Lubricants based on polyethers frequently have, however, only an inadequate stability towards heat and oxidation and therefore do not always meet all the requirements imposed on high-grade lubricants.

Additives, such as aromatic amines, for example phenyl-α-naphthylamine, phenothiazine derivatives and certain diphenylamine derivatives, have already been proposed for improving the properties of lubricants based on polyethers (see Ullmanns Encyclopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 4th edition, Volume 20, page 510, DE-OS (German Published Specification) No. 2,211,805 and DE-OS (German Published Specification) No. 2,806,133.

The known additives which act as stabilizers do not, however, meet all the requirements in respect of stabilization of lubricants based on polyethers. Thus, in spite of this stabilization, oxidative degradation of the lubricants is seen by the relatively rapid decrease in their viscosity. In addition, when the lubricants thus stabilized are used at higher temperatures for a prolonged period, a considerable loss is observed due to the evaporation of volatile degradation products.

Stabilized lubricants based on polyethers have now been found, which are characterized in that they contain diphenylamine derivatives of the formula ##STR3## in which R 1 denotes hydrogen, a straight-chain or branched C 4 to C 12 -alkyl group, a straight-chain or branched C 4 to C 12 -alkylene group, a C 7 to C 12 -aralkyl group, a C 7 to C 12 -aralkylene group, a C 5 to C 12 -cycloalkyl group which is optionally substituted by C 1 to C 6 -alkyl, -alkenyl, -cycloalkyl or -cycloalkenyl, or a C 5 to C 12 -cycloalkenyl group which is optionally substituted by C 1 to C 6 -alkyl, -alkenyl or -cycloalkenyl, but does not denote a hydrocarbon group derived from a terpene, in the ortho- or para-position relative to the particular N atom, n represents an integer from 1 to 29 and Y represents a bifunctional radical --A-- and optionally additionally a bifunctional radical --B--, in each case in the ortho- or para-position relative to the N atoms, wherein --A-- denotes a straight-chain or branched C 4 to C 12 -alkylene radical, a C 7 to C 12 -aralkylene radical, or a C 5 to C.sub. 12 -cycloalkylene radical which is optionally substituted by C 1 to C 6 -alkyl or -cycloalkyl, but does not denote a hydrocarbon radical derived from a terpene, and --B-- denotes --S-- or ##STR4## wherein R 2 represents hydrogen, C 1 to C 6 -alkyl or C 5 to C 6 -cycloalkyl, and wherein --B-- makes up an amount of 0 to 50 mol % of Y.

In formula (I), R 1 preferably represents hydrogen, benzyl, styryl, α-methylstyrl, tert.-butyl, tert.-amyl, isononyl, cyclohexyl, isooctyl, methylcyclohexyl or one of the radicals ##STR5## in each case in the ortho- or para-position relative to the particular N atom, particularly preferably hydrogen, ##STR6## or α-methylstyryl, in each case in the ortho- or para-position relative to the particular N atom, and especially preferably hydrogen, n preferably represents an integer from 1 to 19, and particularly preferably an integer from 1 to 12, Y in the meaing of --A-- preferably represents ##STR7## and in the meaning of --B-- preferably represents --CH 2 --.

The content of --B-- in Y is preferably 0 to 40 mol %. Particularly preferably, Y represents only --A-- in the meaning of ##STR8## The lubricants which are based on polyethers and are stabilized according to the invention can contain diphenylamine derivatives of the formula (I) in amounts of, for example, 0.1 to 10% by weight, based on the mixture. This amount is preferably 0.2 to 5% by weight.

Diphenylamine derivatives of the formula (I) are known per se. They can be prepared, for example, by a process in which diphenylamine and/or diphenylamine derivatives of the formula (II) ##STR9## in which R 1 has the meaning given in the case of formula (I), are reacted in the presence of a strong acid (pK a value of less than 2) at 50 to 300° C. with bifunctional compounds of the formulae (III) to (V)

Hal-Y-Hal (III)

in which Y has the meaning given in the case of formula (I) and Hal represents a halogen atom or with olefines derived from the compounds of the formulae (III) to (V) by splitting off HHal, H 2 O or HCOOH.

Lubricants based on polyethers are also known per se (see, for example, R. C. Gunderson and A. W. Hardt "Synthetic Lubricants", Reinhold Publishing Corp. New York, 1962, pages 61 et seq.). In general, they contain polymers of epoxides of the formula (VI) ##STR10## in which R 3 and R 4 can be identical or different and represent hydrogen or C 1 to C 12 -alkyl, and/or polymers of tetrahydrofuran and/or block polymers and/or random polymers of various epoxides of the formula (VI) and/or tetrahydrofuran. The lubricants based on polyethers preferably contain homopolymers and/or copolymers of ethylene oxide and propylene oxide.

Such polyethers are in general oils which have molecular weights of, for example, 500 to 20,000. They preferably have molecular weights of 1,000 to 7,000. They can be prepared by methods which are known per se, for example by alkali-catalysed or acid-catalysed polymerization of the cyclic ethers, it being possible to use monoalcohols, polyalcohols or amines as starter molecules. The OH end groups of the polyethers thus prepared can be blocked, that is to say converted into ether or ester end groups, by customary alkylation or acylation.

Polyethers which are particularly suitable for the stabilization according to the invention are oils with molecular weights of 500 to 20,000, preferably with molecular weights of 1,500 to 7,000, which have been prepared by alkali-catalyzed addition of propylene oxide and, if appropriate, ethylene oxide onto alcohols, for example onto ethanol, butanol, propanediol, glycerol, trimethylolpropane or pentaerythritol. Other polyethers which are particularly suitable for the stabilization according to the invention are oils with molecular weights of 500 to 10,000, preferably of 1,000 to 7,000, which have been prepared by acid-catalyzed copolymerization of at least two substances from the group comprising ethylene oxide, propylene oxide, tetrahydrofuran and epoxides with 6 to 20, preferably 8 to 12, C atoms.

The polyethers stabilized according to the invention with diphenylamine derivatives of the formula (I) can additionally contain other constituents, for example

(a) Diphenylamine derivatives of the formula (VII) ##STR11## in which R 5 and R 6 can be identical or different and represent hydrogen, C 1 to C 12 -alkyl or C 7 to C 12 -alkaryl, preferably isooctyl and/or styryl, Z represents sulphur and m represents 0 or 1, (b) metal deactivators of the salicylideneamine type of the formula (VIII) ##STR12## in which X represents a two-bonds, optionally nitrogen-containing aliphatic radical, preferably ##STR13## where m=0 to 5, preferably where m=1,

(c) Other stabilisers based on aromatic amines, such as phenyl-α-naphthylamine, 4-isopropylaminodiphenylamine, N,N'-dicyclohexyl-p-phenylenediamine and/or 4-isohexyl-amino-diphenylamine,

(d) Phenolic antioxidants, such as 2,6-di-tert.-butyl-p-cresol, 2,6-di-tert.-butyl-4-methoxypehnol, methyl- and butyl-hydroquinone, 2,2'-methylene-bis-(6-tert.-butyl-4-methylphenol), 2,2'-methylene-bis-(6-cyclohexyl-4-methylphenol), 1,1'-bis-(2-hydroxy-3,5-dimethylphenyl)-butane, 2,2'-thio-bis-(4-methyl-6-tert.-butylphenol) and 4,4'-methylene-bis-(2,6-ditert.-butylphenol), and

(e) Other customary additives, such as corrosion inhibitors, for example succinic acid half-esters, phosphorous acid esters and phosphoric acid esters, and anti-wear agents, for example nitrogen and/or phosphorus compounds, such as triaryl phosphates and mono- and di-arylphosphonates.

The diphenylamine derivatives of the formula (VII) can be present, for example, in amounts of 0 to 10% by weight, based on the mixture. This amount is preferably 0.1 to 5% by weight.

The metal deactivators of the formula (VIII) can be present, for example, in an amount of 0 to 4% by weight, based on the mixture. This amount is preferably 0.5 to 2% by weight.

The other additives can in each case be present, for example, in amounts of 0 to 10% by weight, based on the mixture. This amount is preferably 0 to 5% by weight.

Stabilized lubricants based on polyethers, which simultaneously contain diphenylamine derivatives of the formula (I), diphenylamine derivatives of the formula (VII) and metal deactivators of the formula (VIII) are of particular interest, since these give a synergistic mixture, that is to say a mixture which is better stabilized than can be expected from the individual actions of the additives.

The lubricants according to the invention can be prepared in various ways. For example, diphenylamine derivatives of the formula (I) and, if appropriate, other additives can be added in the desired proportions to polyethers and, if appropriate, dissolved therein by heating, for example to 80° to 100° C. Another possibility comprises first preparing concentrates of the stabilizers with a suitable medium and adding these, according to the desired proportions, to the polyether lubricant to be stabilized. Suitable concentrations for such concentrates are, for example, from 20 to 90% by weight, and suitable media for their preparation are, for example, higher alcohols, such as 2-ethylhexanol and octaethylene glycol, and especially polyether lubricants. It is also possible first to mix the stabilizers or their concentrates in the desired proportions and to add this mixture to the lubricant to be stabilized.

The drop in viscosity and the losses by evaporation of volatile degradation products is substantially lower with the lubricants which are based on polyethers and are stabilized according to the invention with diphenylamine derivatives of the formula (I) than with lubricants which are based on polyethers and have been stabilized with customary additives.

The following examples illustrate the present invention without limiting it.